Climbing rungs for anchoring to the wall surface of a manhole shaft are known, employing a generally U-shaped aluminium or steel rung consisting of two parallel legs for anchoring in the wall spaced apart by a central tread. The two parallel anchoring legs are to be permanently inserted into receptive bore holes in the wall.
It is recognized by those skilled in the art that a manhole shaft is a corrosive environment presenting the risk of rusting or corrosion of steel or aluminium climbing rungs when they are anchored into a wall. Accordingly a variety of coatings and combinations thereof have been proposed for protecting the aluminium or steel `core` of the step against corrosion. For example the `core` may be sheathed in a pre-formed protective jacket or sleeve open at both ends. A pair of pre-formed protective anchor sleeves, each one sealed at a conically tapered end for insertion into a receptive bore hole in the wall are adapted to fit over the ends of the anchoring legs and overlap the protective jacket sheathing the `core` of the climbing rung. Exposure of the `core` to corrosive substances is thereby mitigated. However a perfect seal is not achieved--use of the climbing rung will result in flexion and vibration spreading corrosive substances along the cylindrical seams between the anchor sleeve, protective jacket or sleeve and `core`. The resulting rust and corrosion underneath the anchor sleeves and protective jacket is not visible to inspection.
Many building codes now require that protective coatings provide a bond encapsulating the `core` of the climbing rung sufficient that current will not be conducted from the `core` through the protective coating to electrical ground. The response to the need for a more effective means of sealing the `core` has been to encapsulate it in a corrosion resistant material such as recovered battery case material by means of an injection moulding process. According to this process the entire climbing rung is placed in the cavity of a mould body and molten polyethylene or the like is injected into the mould cavity. Usually the `core` is hollow inside. It is therefore necessary to plug the ends of the `core` to prevent the molten polyethylene from being injected into the `core`. Since any contact between the climbing rung and the side of the mould cavity will preclude the injected polyethylene from forming a coat of uniform thickness it is necessary to support the climbing rung centrally in the mould cavity. To this end, typically pins are provided which project into the mould cavity to support and evenly space the climbing rung from the surface of the mould cavity. After the climbing rung is ejected from the mould body, holes are left in the coating proximate the site where the support pins have contacted the `core`, which must be detected and filled. Plugging the ends of the `core` and filling the pin holes adds to the cost of, and time needed, for production. In another embodiment it has been taught to provide support pins that `float` in bore holes in the body of the mould under air pressure less than the injection pressure and which normally project into the mould cavity. When the ejection half and the injection half of the mould are joined, the support pins extend into the mould cavity to centrally support the climbing rung. As the injection pressure becomes greater than the air pressure `floating` the support pins in the bore holes, the pins retract from the mould cavity into the body of the mould. Since polyethylene can leak into the bore holes under pressure, the fit of the support pins in the bore holes requires a tolerance of 0:0. Moulds having such tolerances are expensive to manufacture. In practice the pins get stuck and fail to retract. Therefore a hole may unexpectedly be left in the injection moulded coating which must be detected and filled, resulting in high rejection rates and low productivity. Accordingly, in practice such a system has an extremely low production rate and is prone to failure.
It is therefore an object of this invention to provide an improved climbing rung having a combination of protective coatings sealing the `core` sufficient that electrical current will not be conducted through the protective coatings to electrical ground.
It is a further object of this invention to provide an improved method of encapsulating the core of a climbing rung in an injection moulding process.
Further and other objects of the invention will be apparent to those skilled in the art from the following Summary of the Invention and Detailed Description of Embodiments thereof.